This invention relates to processes and apparatus used for cleaning tubes, particularly tubes of a heater.
Heaters are used in petrochemical installations to heat fluids for a variety of purposes, typically to break apart larger hydrocarbon molecules into smaller molecules. The heaters contain tubes, up to and even more than a kilometer long in each of several passes, that pass first through a convection section of a heater and then through a radiant section. During use, the heater tubes gradually become contaminated on their insides. This contamination, typically coke, tends to degrade the efficiency of the heater over time and can eventually cause the heater to stop working.
Various methods are known for decoking heaters. In one method, the heater is shut down and steam cleaned with high pressure steam. In another method, described for example in U.S. Pat. No. 5,358,573 issued Oct. 25, 1994, by the same inventor, the heater is shut down and pigs with appendages run through the heater until it is clean. In another method, described in U.S. Pat. No. 5,186,815 issued Feb. 16, 1993, the heater tubes are treated while the heater is in operation by injecting solid particles of very small size into the heater tubes, recovering the solid particles at the outlet and recirculating the solid particles back to the inlet of the heater.
Use of pigs to clean heater tubes is very effective since the pigs have a robust scraping action. Heater operators in South America who have used the inventor""s method described in U.S. Pat. No. 5,358,573 have asked the inventor to provide cleaning of the heater tubes by pigs while the heater is in operation. Since in many heater tubes temperatures are far higher than conventional polymer pigs will withstand, the inventor has identified a need for a new pig for cleaning an operating heater, and a method for its use. The inventor has thus come up with a novel solution to the problem of providing a heater cleaning operation by using pigs while a heater is in operation.
It is an object of this invention to provide a novel pig and process for pigging tubes, as for example tubes of a heater, even while it is operating.
There is therefore provided in accordance with an aspect of the invention, an improved pig made from a body, preferably hollow, circular at least in one cross-section to fit within a tube, with scraping edges on the outer periphery of the body. Preferably, the scraping edges are the longitudinal edges of a wire. The wire may be in the form of a tubular mesh, which may be knitted or woven or knotted. The pig is preferably radially expandable up to twice its fully compressed radius, and may have an expander to force it radially outward. The pig is preferably made of a resilient wire having a polygonal cross-section. The pig is preferably entirely made of metal.
Such a pig is capable of cleaning operating heaters without immediate degradation, and is capable of cleaning operating heaters having variably sized tubes.
According to an aspect of a method of the invention, there is provided a method of cleaning tubing comprising the step of running a pig having a scraping action through the tubing, wherein the scraping action is caused by scraping edges on the outer periphery of the pig.
According to further aspects of the method of the invention, the pig has one or more of these characteristics: hollow, metallic, formed of a tubular mesh, and having scraping action caused by edges, preferably longitudinal edges, of a wire.
According to a further aspect of the method of the invention, the heater is cleaned while it is operating.
According to a further aspect of the method of the invention, the pig is run through the tubing repeatedly.
According to a further aspect of the method of the invention, the pig is run through the tubing after contaminant has formed on the inside of the tubing but before the contaminant has hardened.
According to a further aspect of the method of the invention, the tubing is first thoroughly cleaned by a pig, as for example a polymer pig with embedded metallic scraping elements, with a robust scraping action.
In one aspect of the method of the invention, as the pipe pig progresses from smaller to larger tubes, the pig radially expands within the tube, while maintaining 360xc2x0 C. cleaning coverage of the tube.
These and other aspects of the invention are described in the detailed description of the invention and claimed in the claims that follow.